On some archs uclibc does not provide the ucontext structure despite
providing ucontext.h, for details see
https://git.buildroot.net/buildroot/commit/?id=f1cbfeea95e6287c7a666aafc182ffa318eff262

This patch improves the detection of ucontext by making sure that
HAVE_UCONTEXT_H is only set when makecontext() was found.
Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>